This invention relates generally to an apparatus for capping and sealing the end of a pipe, tube or cylinder during hydrostatic testing. More particularly, this invention concerns an apparatus having a plurality of jaws pivotally mounted on a plurality of arms, and the arms are in turn pivotally mounted to a body adapted to axially receive a pipe, tube or cylinder. With jaws of an area determined in accordance with the present invention the mechanical pressure upon the outer wall of the pipe substantially equals or is proportional to the fluid pressure upon the inner wall of the pipe. Therefore, the wall of the pipe is compressed; the outer diameter of the pipe is not substantially stressed. The apparatus is adapted to cap the pipe during hydrostatic testing without generating significant hoop stresses upon the pipe itself.
While the present invention is described with reference to capping a pipe, it is intended that "pipe" wherever used herein shall include tubing or other cylindrical objects. The invention may also facilitate grasping a cylindrical object for other purposes.
Hydrostatic testing of pipes, upset tubing, and other cylindrical objects is necessary to insure that the pipe or tubing will withstand pressure levels equal to or greater than those which are expected to be encountered during use. Hydrostatic testing is generally a requirement of the American Petroleum Institute (A.P.I.) for most types of pipes.
In the absence of adequate hydrostatic testing during the installation of a petroleum or oil by-product pipeline, hidden flaws in the pipe may cause it to burst. An erupted pipe may go undetected for relatively long periods of time and loose a significant portion of its contents, which may have adverse consequences for the environment. In addition, the adverse economic consequences involved in repairing a broken pipeline and the consequential down time can be severe. The resultant disruptions in the supply of oil, natural gas, or other commodity intended to be transported through the pipeline, in addition to the foregoing, require pipe and tubing to be adequately tested hydrostatically prior to or during the installation of such pipeline.
Manufacturing operations for producing pipe or tubing expected to withstand pressure during use require hydrostatic testing as a quality control measure. Unless pipe and upset tubing are tested, undetected flaws can create serious safety hazards to an end user. Therefore, hydrostatic pressure testing has become a practical necessity for pipe, fittings and tubing after fabrication.
In the past, it has been common to weld a cap onto the end of a pipeline or an unthreaded or unflanged pipe to be tested. Welding requires expensive skilled labor to perform the welding operations. This work cannot ordinarily be performed by unskilled laborers. The cap must be securely welded to withstand test pressurization without blowing off of the end of the pipe. In some cases, stress relieving and X-raying of the welded cap is required. After testing, the cap must be cut off of the pipe. Not only is welding time-consuming and expensive, but also, the danger of explosion in some environments may be so great that welding operations are not feasible.
Prior art devices have included caps adapted to be screwed onto the end of the pipe or tubing to be tested. Such devices utilize the threads of the pipe to secure the device to the pipe. It is believed that API specifications require that such devices be tightened hand-tight only. Otherwise, the threads of the pipe may be damaged. However, a hand-tight cap will not withstand high pressure testing. Thus, to achieve a satisfactory seal, such devices are often overtightened resulting in stripped threads and damage to the pipe or fitting. Threads unknowingly stripped during installation present a latent danger that can kill or injure if pressurization causes the cap to blow off of the end of the pipe during testing. Such devices may not have the structural integrity to withstand testing pressures and may be blown off, thus presenting a serious threat of injury.
Moreover, such devices must be tightened under substantially zero internal pressure conditions because tightening during pressurization is impractical and dangerous. During zero internal pressure conditions there is no internal fluid pressure to offset the clamping pressure. Thus, the threads and other portions of the pipe must be stressed, and damage to the pipe or threads may result. Clearly, such prior art devices are unusable on pipe with damaged threads or no threads at all. It is often impractical to machine new threads onto the end of the damaged pipe because of the costs and delay involved. Thus, the cost of pipe ruined by such devices renders these known devices impractical in many cases.
In order to safely perform a hydrostatic test, all air and other gaseous matter must be expunged from the inner volume of the pipe. Failure to remove substantially all of the air creates an explosion hazard that can pose a serious danger to anyone in the vicinity of the testing apparatus. Most prior art devices fail to have a positive safeguard against such trapped air type explosions.
For example, in the past it has also been common to attach a cap onto the end of a pipe with bolts, screws, or other fastening means. Such methods of capping a pipe have been unsatisfactory, however. Not only are such methods time-consuming, but injuries and even deaths can result from such caps being blown off of the pipe during high pressure testing, because such methods and apparatus fail to provide a satisfactory means for eliminating air or gas from the interior of the pipe.
Known methods of removing air from a pipe include tilting the uncapped end of a joint of pipe in an upward direction, thereby causing air bubbles to migrate to the raised end of the pipe. Such tilting methods often result in pipe handling problems because the pipe may slip or be dropped. The expense and time required for such handling methods, in addition to the hazard posed when such pipe is dropped or slips, renders such methods unsatisfactory. Moreover, long sections of pipeline cannot be conveniently tilted or may be too long for the pipe handling apparatus available.
Another example of a prior art mechanism utilizes a set of independently operated jaws. Each jaw is manually tightened against the pipe by means of a screw or bolt which is adjusted with a wrench to jam the jaw against the pipe wholly independently of the other jaws. This type of mechanism is unsatisfactory at least insofar as it may create hoop stresses or deformations upon the pipe. For high pressure testing, the jaws must be tightened to a holding force sufficient to withstand test pressure before the pipe is pressurized. This imposes excessive stresses on the walls of the pipe which are likely to overstress, deform or weaken the pipe.
Other devices employ hydraulic rams and threaded devices to slips which apply an outside force to hold the cap onto the pipe. Necessarily, such devices must stress the outer wall of the pipe while there is little or no internal pressure to offset the pipe gripping force. If the initial gripping force is inadequate, the cap will slip off during internal pressurization of the pipe. Such known devices have a tendency to deform or damage the pipe.
Representative prior art patents illustrating problems of the type overcome by the present invention are U.S. Pat. Nos. 2,699,802; 3,647,108; 3,765,560; 3,885,521; 1,746,071; 2,399,544; 2,445,645; 2,480,358; 2,851,061; 3,108,012; 3,125,464; 3,525,111; and 3,703,947.
U.S. Pat. No. 4,077,250, granted to applicant herein, discloses a pipe closure apparatus having gripper means connected to a spline or rib by means of a rotatable link attached to a mounting pin. This patent failed to address the problem of removing air from the pipe in order to reduce the hazard of explosion. Moreover, this patent fails to address the problem of extrusion of the seal during high pressure testing.
The eariler patent disclosed an alignment ring positioned axially to the rear of a gripper means. The rotatable link was attached to a spline or rib, which was in turn joined to the body or closure plate. However, this arrangement was found to be unsatisfactory in some instances. When pressure is introduced into the pipe, the gripper means is urged in a direction generally toward the rear of the body. The gripper means therefore urges the spline or rib generally radially outward. Difficulties were encountered in manufacturing a commercially practical spline or rib adequate to withstand the radially outward force generated by the gripper means during high pressure testing. It was found that an enormous rib was required to withstand high pressure testing because of the manner in which the load was transmitted to the rib in the earlier patent.
The prior art patent also has failed to address the problem created by dirt, grime or air which may become entrapped within a U-shaped seal. The entrapment of air, dirt, debris or other foreign matter within a seal may inhibit, if not render inoperative, the intended operation of the seal. Nor did the earlier patent have a pre-loaded lip to assure zero leakage during low pressure filling of the pipe.
The earlier patent was not adapted for testing pipe with upsets, bell ends or coupling ends. In order to pass such pipe ends, the alignment ring had to be made too large to effectively align the pipe during testing. That patent had no centralizer means for centering such a pipe after passing the larger end of the pipe.
The earlier patent teaches the use of a single rotatable link joining a gripper to a rib. A single link has proved to be unsatisfactory in some instances. A single link introduces stability problems and may create undesirable stresses upon the diameter of a pipe and the rib. Moreover, the earlier patent fails to address the unexpected uneven distribution of the load forces upon the pipe achieved by the gripper means disclosed in that patent where the gripper means is not permitted to move generally parallel to the axis of the pipe.
The prior patent was not adaptable to test several different pipe sizes with a single apparatus. The prior patent failed to provide means for readily adapting the apparatus to fit different pipe sizes.
While prior art arrangements have exhibited a degree of utility in capping the end of a pipe or tube to permit hydrostatic testing, room for significant improvement remains. The problems enumerated in the foregoing are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known apparatus for capping pipes. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that the prior arrangements appearing in the art have not been altogether satisfactory.